Transmitter for my car

[vaineo]

I wanted to build a transmitter for my discman for my car. I don't have any variable capacitors so I made a small transmitter design with a fixed capacitor. It should transmit at about 88.3Mhz but I wanted some people with more experiance with this to check out my schematic before I build it to see if it might work. Heres the schematic.

 

[Klaus]

So, you want to transmit the headphone output from your portable CD player so you can hear it from the car speakers via the FM radio?
I do that via a simple cable :wink: but perhaps your car stereo has no external input for that.
You really have to be able to tune the transmitter frequency so it is received at a clear spot on the dial. You might end up on top of a station otherwise.
Small trim capacitors are available, might be best to get one and try it then.
Keep all the connecting wires near the cap and the coil as short as possible if this is not on a ready made PCB.

 

[Nigel Goodwin]

You appear to have made a mistake, you have the base of the transistor shorted to the 3V supply - it should go via a resistor.

 

[vaineo]

Like you said, I don't have a spot for an external input with my radio.

If i add a resistor between the positive and the base should I change the supply to 9v instead?

 

[Russlk]

I tried simulating your circuit, I was not suprised that it did not oscillate. Try this mod:

 

[vaineo]

Is there any other way to tune the circuit without using a variable capacitor? I know stretching out the coil will change the inductance but I don't know how much or how precise that will be.

I've been looking at some of the other types of oscillators and I saw some that used a crystal. Have you tryed those before, if so does the crystal provide the tuning for the circuit?

I don't really want to use a pre-desined device, i'm trying to figure out how to build one on my own, but as you can see, its not working as good as I hoped.

 

[Russlk]

A crystal will give a precise frequency, but it cannot be Frequency Modulated, so it won't be any good for FM. Why not build a kit? Jameco has a stereo kit for $29.95 or mono kit for $12.49.

 

[vaineo]

I could buy a kit, but i've built one of those before. Since it doesn't really matter when i get this finished i wanted to try and design it myself see if i could get one to work. Wanted to learn the concept of it and this seems like the best way.